The present invention is in the field of single lens reflex cameras and in particular is directed to a single lens reflex camera with interchangeable mirror operation to best suit the particular photographing conditions.
It is well known to provide a single lens reflex camera with many features so systematized that it can allow a variety of photographies with the aid of various techniques. However, the single lens reflex cameras have not been developed to such a level as to perform perfect functions under special conditions. The portions of the single lens reflex camera which especially need further development include the finder optical system and the photometric optical system.
FIG. 1 shows an optical system of a conventional single lens reflex camera, comprising an interchangeable lens system 1, a retractable reflecting mirror 2, film 3, focusing plate 4, pentagonal prism 5, and magnifying lens 6. Generally speaking, the locus of the leading end of a reflecting mirror, which is made to pivot, is restricted by the back focus of the photographing lens system. This restriction is due soley to the need to avoid having the mirror strike the lens as it swings from the prephotography position, shown in the drawing, to the photographing position, e.g. parallel to the focusing plate. The back focus is typically shorter for a wide angle lens (having a short focal length) and longer for a tele-photo lens (having a long focal length). To cope with the restriction, therefore, the reflecting firror is made sufficiently short so as to cope with the interchangeable lens system having the shortest focal length.
However the latter shortening of the mirror, to accomodate this shortest focal length lens system for which the camera is designed, results in the so-called "mirror shortage" phenomena. This phenomena is observed through the finder optical system, 4, 5 and 6 as a darkening of the leading edge 4a of the focusing plate, particularly when a large focal length lens system, such as a telephoto lens, is used. This is caused by the fact that the light which would normally reflect off the edge of the mirror, is not reflected by the shortened mirror.
Another feature of SLR retractable mirror, which feature becomes a problem in certain types of photography operations is the shock accompanying the retraction and restoration of the mirror from the optical path just prior to and just after the shutter open period. In certain situations the shock results in undesireable blur and noise. One known technique for overcoming the effect of the shock on picture taking is to provide a separately actuable mechanism for holding the mirror out of the optical path.
A translucent reflecting mirror will also prevent shock, and its effects, because such a mirror does not have to be moved. It remains in the optical path, reflecting a portion of the light to the finder system and passing a portion of the light to the film. Unfortunately, however, in most photography situations the translucent mirror is not suitable as it can produce a double or flared image, thus degrading the quality of the picture.